Acetic acid production by way of methanol carbonylation is well known in the art. Generally speaking, a methanol carbonylation production line includes a reactor, a flasher, purification and recycle. In the reactor section, methanol and carbon monoxide are contacted with rhodium or iridium catalyst in a homogenous stirred liquid phase reaction medium in a reactor to produce acetic acid. Methanol is pumped to the reactor from a methanol surge tank. The process is highly efficient, having a conversion of methanol to acetic acid of typically greater than 99 percent. A flash vessel coupled to the reactor flashes a draw stream in order to remove crude product from the reaction mixture. The crude product is fed to a purification section which includes generally a light ends or stripper column, a drying column, auxiliary purification and optionally a finishing column. In the process, various vent streams containing light ends, notably methyl iodide, carbon monoxide and methyl acetate are generated and fed to a light ends recovery section. These vent streams are scrubbed with a solvent to remove the light ends which are returned to the system or discarded.
It has been noted in various references that flash vessels used in carbonylation production processes may or may not be heated. See U.S. Pat. No. 5,874,610 to Clode et al. at Col. 2, lines 20-54; U.S. Pat. No. 5,750,007 to Clode et al. at Col. 2, lines 40-51; and U.S. Pat. No. 5,990,347 to Clode at Col. 2, lines 50-57. See also, U.S. Pat. No. 6,066,762 to Yoneda etal. which discloses a flash temperature of from 80° C.-180° C. (Col. 16, lines 40-44). It has not been appreciated, however, that temperature control within a relatively narrow window can be used to greatly increase the acetic acid content of the crude product stream in an acetic acid process. In conventional systems, flashing is typically carried out adiabatically and there is a large temperature drop relative to the feed stream because of the heat of vaporization of the crude product.